1. Origin of the Invention
The invention described herein was made in the performance of work under a NASA Contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 STAT 435; 43 USC 2457).
2. Field of the Invention
This invention relates to electric discharge gas lasers and, more particularly, to a system incorporating a plurality of pulsed electric discharge gas lasers.
3. Brief Description of the Prior Art
U.S. Pat. Nos. 4,088,965 and 4,275,317 and U.S. patent application Ser. No. 727,931, filed Apr. 29, 1985, entitled "Magnetically Switched Power Supply System for Lasers," assigned to the assignee of the present application, describe high-power electric discharge gas lasers, power supplies and transmission lines for supplying excitation energy to such lasers. The technology disclosed in the patents and application permit more reliable operation with lower system costs at higher average power, higher pulse energies and higher repetition rates than theretofore possible. However, there are practical limiting factors to the power and repetition rate achievable with a laser in a commercial environment.
Specific areas of concern in designing a laser, such as a rare gas excimer laser, for providing high pulse and average power include: (1) the physical size of the laser; (2) the need for high voltage pulsed power supply to scale a single device to high pulse energy; (3) the high speed recirculation of a self-contained gas mixture in order to operate at high repetition rates; (4) the need to have large, expensive optical elements for transmitting and directing the laser beams from a large scale excimer laser; and (5) component lifetime.
The prior art has demonstrated that efficient lasing on a variety of excimer gas mixtures, i.e., xenon chloride (XeCl), krypton fluorine (KrF), argon fluorine (ArF), etc., can be obtained at a maximum specific output energy density in the range of 0.5 to 1.5 Joules per liter-atmosphere. Therefore, to scale the energy per pulse from a single laser to the multiple Joule/pulse level, the discharge volume and/or the pressure has to be increased. Increasing the laser discharge volume and pressure requires an increase of the gas breakdown and sustained discharge voltage necessary to excite the laser (e.g., the breakdown voltage is approximately three to five times the sustained discharge voltage). For example, a 5.times.5.times.50 cubic centimeter (cm.sup.3) discharge volume XeCl laser at five atmospheres ("atm") producing five Joules per pluse has a breakdown voltage of about 75 kilovolts ("kV") and sustained discharge voltage of about 30 kV with a helium buffered gas mix and about 60 kV breakdown voltage and a 20 kV sustained discharge voltage with a Neon buffer. To scale a XeCl laser to produce about 14 Joule/pulse would require a discharge volume of about 7.times.7.times.80 cm.sup.3, a breakdown voltage of about 125 kV and a sustained voltage of about 40 kV for a buffered XeCl gas mix. Further extensions of the output pulse energy from an excimer laser would require even larger discharge volumes, higher voltages and energy, as well as larger optics.
The problem of providing adequate gas flow within an excimer laser to enable high average power operation, without the loss of optical beam quality and pulse energy, also becomes more aggravated as the firing or repetition rate is increased. Since electric discharge gas excimer lasers have low efficiencies, i.e., 1-4%, most of the discharge energy goes into heating of the gas mixture. In most cases, this heat must be removed from the recirculating gas mixture to maintain efficient laser operation. Also, where high repetition rates are to be encountered, the flow system must be carefully designed to provide efficient gas exchange (several clearings of the discharge region between laser pulses) and to control flow turbulence and discharge acoustic shock waves. Flow disturbances cause density gradients in the gas flow which can severely degrade the optical beam quality as well as the energy output from the laser. Increasing the repetition rate and average power in a laser requires a higher gas flow velocity to maintain the same flush factor through the electrodes and a larger heat exchanger to remove the excess heat from the gas. The flow turbulence and acoustic waves increase with higher gas flow rates and increased energy loadings.
In addition to the above problems, the replacement of power supply componenets becomes an increasing problem as the repetition rate is increased. While the saturable inductor switches described in U.S. Pat. No. 4,275,317 have an almost unlimited life, other power supply components do not. Primary switches such as thyratrons have lifetimes of the order of -10.sup.9 shots, capacitors and laser electrodes have similar lifetime restrictions due to the number of accumulated discharge cycles. When the pulse rate is increased, the operational lifetime of these electrical components becomes shorter, so replacement and maintenance schedules are shorter and there is more frequent down time for the laser.
A single excimer laser designed to provide high average power at high repetition rate operation will not only result in a complex, expensive device that is costly to maintain, but also in a device which is not suited for many practical applications. A variety of applications require not raw energy from the laser, but energy in a particular form, e.g., a particular wavelength; a certain repetition rate or pulse sequence, either high peak power or the same pulse energy but in a longer pulse width. The single high energy laser design limits the applicability of the laser to only a few specific tasks.
For example, a single excimer laser has maximum output pulse width limited to the nanosecond ("ns") range (e.g., 10-1000 ns) because all of the halogen donor reacts in this time period. Applications such as metal welding may require pulsewidths of many microseconds (.mu.s) to effectively heat and melt the surrounding area. Other applications such as solar cell annealing appear to require a high energy short duration pulse (50 ns) from a laser in the ultraviolet wavelength. A typical solar cell has an area of 100 cm.sup.2 and therefore requires a 150 Joules/pulse excimer laser output with a beam cross section of 10 cm.times.10 cm and pulsewidth of about 50 ns to anneal the cell with a single pulse. To achieve this from a single laser would present an extremely difficult engineering challenge.
While the prior art has proposed the use of several lasers to overcome the limitations of solid state (e.g., ruby laser) or CO.sub.2 laser devices instead of a single laser for specific applications, such prior art devices have not overcome the above problems. For example, U.S. Pat. No. 4,230,993 discloses several series-connected lasers excited by a common power supply and focused on a common point for providing a higher level of energy per pulse or a higher pulse repetition rate. The use of a single power supply for high repetition rate operation has the disadvantages discussed above; i.e., short lifetime for the electrical components and a frequent replacement and maintenance schedule. Also, the use of a single power supply which is alternately connected to the lasers limits the time between laser firings to accommodate the charge built up in the energy storage devices, such as capacitors. This precludes the use of such a system in an application where a long continuous pulse is required; e.g., in welding metal, as discussed above. Further, the use of a series connection and a common focus point for the lasers severely limits the versatility of the system. Recent experiments in material ablation indicate that only a small specific energy is absorbed in cutting certain material such as leather and biological tissue. Increasing the laser energy does not necessarily increase the cutting rate. Focusing several single lasers at a common point, as is described in U.S. Pat. No. 4,230,993, will not increase the cutting rate for such material. Increasing the individual repetition rate may increase the cutting rate, but only at increased cost and decreased time between maintenance.
The above shortcomings of the prior art lasers are overcome by the present invention.